Borehole well logging systems that emit bursts of high energy radiation (neutrons) are routinely used in geophysical exploration for hydrocarbons. Some of the earliest commercial pulsed neutron logging systems differentiated between saline formation liquid from non-saline liquid, assumed to be hydrocarbon. Chlorine in saline water has a relatively large thermal neutron absorption cross-section, while carbon and hydrogen in hydrocarbons have relatively small thermal neutron cross-sections. The decay rate of thermal neutrons is measured between bursts of neutrons by measuring capture gamma radiation as a function of time. This decay rate is, therefore, indicative of the thermal neutron capture cross-section of the borehole environs. Those of ordinary skill in the art will recognize that this decay rate quantity is commonly referred to as “sigma” (Σ). See, e.g., U.S. Pat. No. 7,999,220 to Odom, entitled “Borehole Measurements Using a Fast and High Energy Resolution Gamma Ray Detector Assembly,” which patent is hereby incorporated by reference herein in its entirety.
In the prior art, it is customary for the decay waveform (sigma) to be determined by defining a plurality of time intervals or “gates” during which periods the gamma radiation resulting from neutron capture in the borehole environment is detected. See, for example, U.S. Pat. No. 7,201,125 to Samworth, entitled “Method of Logging a Borehole,” which reference is hereby incorporated by reference herein in its entirety. The output of the gamma radiation detector is integrated during these gating periods to recreate an analog waveform reflecting the measure of decay of the gamma radiation and hence of neutron capture in the formation. See also, U.S. Pat. No. 7,139,350 to Tiller et al., entitled “Method and Apparatus for Measuring Radiation in a Borehole,” which reference is hereby incorporated by reference in its entirety. The Samworth '125 patent observes that by integrating the gamma radiation detector output over gating periods, “there is no need to detect and separate each individual detector pulse.” (Samworth '125, col. 3, lines 19-21).
On the other hand, by limiting the analysis of the sigma curve to a limited plurality of gating periods, a less than complete picture of the actual decay curve can be attained.